Radio receiving apparatus



Jam. 14, 1936. J. w. ALE XANDER RADIO RECEIVING APPARATUS Filed. Feb. 19, 1951 n Ir lNVENTOR JOHN! W\LLEM ALEXANDER BY 4W ATTORNEY Patented at. 14, 1936 UNITED STATES amen PATENT OFFICE RADIO RECEIVING APPARATUS Johan Willem Alexander, Eindhoven, Netherlands, assignor to Radio Gorporation of America, a corporation of Delaware 5 Claims.

The present invention relates to amplifiers and the like and in particular to radio receivers energized from commercial power lines or networks.

In such systems, it is a known fact that ground connections are made and in some instances the ground connection of the power supply system and the ground connection of the receiver are at slightly different potentials.

To be more specific, it is supposed that the cathode of a receiver tube is to be grounded. For this purpose it is customary to connect to a convenient radiator or water pipe. -However, these connections are not ideal since there is enough impedance therein to cause the cathode to be at a different ground potential with respect to the power supply network ground, hence, noises and various other disturbances are impressed upon the grid circuit of the amplifier tubes.

Especially in cases where the earth-connection of the radio set is bad, the so-called networkdisturbances may penetrate to the grid circuit of one of the tubes, for example, via the antenna.

According to the invention the grid-cathode circuit of at least one of the space discharge tubes and in particular that of the tube which is directly coupled with the antenna, is inserted in the diagonal of a bridge connection of which the other diagonal is arranged between the earth and the cathode of the next tube.

Indeed, it has been found, that a great part of the disturbances caused by the network may reach the grid circuit of one or more of the tubes due to certain unsymmetries in the circuit arrangement. When these disturbances reach the grid of one of the tubes, they are amplified in the next amplification stages, thus giving rise to disturbing noises. I

In the accompanying drawing Fig. 1 shows a diagram of one of the most usual receiving circuit-arrangements. By means of this diagram will be explained how the network-disturbances may reach the grid circuits of one or more of the tubes,

Fig. 2 shows an embodiment of the invention; and,

Fig. 3 shows a balanced bridge circuit illustrating the principles of the invention.

In Fig. 1 the antenna (aerial network) is denoted by I. In the case under consideration this antenna may be considered as a capacity relatively to earth. The condenser by which the capacity of the aerial network I relatively to earth may be replaced is designated by H. The aerialconductor i is connected to the grid of a discharge tube 4 across a small condenser 2. The grid circuit of this tube is termed by the tunable circuit 3. The cathode of G is connected to earth across an impedance Ml. As a rule this impedance in will preferably be kept as low as possible, which may be accomplished by using an earth connection which is as short as possible and which is free from resistance and self-induc- 7 tion. As a rule, however, it will not be possible to construct the earth connection in such manher that it is entirely free from resistance. or from self-induction. In Fig. I this unavoidable impedance prevailing in the earth-connection is denoted by Ill. The plate of the tube 4 is connected in the usual manner to the grid of a detector valve 5. The grid condenser is designated by l and a leakage resistance by 8. 6 denotes a tuned circuit serving for transmitting the amplified high frequency oscillations to the grid of the tube 5. 9 denotes the loud-speaker which is inserted in the plate circuit of the detector and i2 and I3 designate the terminals of the source of plate current connected to the lines 14 and I5.

It has been found, that network disturbances may be produced at the ends of the impedance m which are probably due to unavoidable capacltive connections. Indeed, the network is always connected for alternating current to the filament circuits and on the other hand earthed, either directly or capacitively. Due to this a disturbing voltage- V1'may be set up between the points I8 and I9, which directly acts upon the impedance It! as appears from Fig. 1. Consequently, in the circuit constituted by l0,'3, 2, II and earth a disturbing voltage is set up which is transmitted to the grid of the tube 4.

Fig. 2 shows a circuit arrangement by which these disturbing voltages are eliminated. The difference between this circuit arrangement and that shown in Fig. 1 consists only in that the grid of the tube 6 is connected to the impedance 22 across a small condenser H5, whereas the filament is connected to an intermediate point of the impedance 22. Since as a rule the impedance HI shown in Fig. 1 cannot be modified, a resistance or similar impedance may be connected in series with this impedance Ill. This purposely arranged impedance together with the impedance l0 shown in Fig. l constitute the impedance 22 shown in Fig. 2. The circuit arrangement shown in Fig. 2 may be readily obtained Irom that shown in Fig. 1 by cutting the filament conductor 20 shown in Fig. 1, at 2| and by inserting a resistance or other impedance in between,whereupon one end of this impedance is connected to the grid via the condenser l6.

By a proper choice of the value of this auxiliary impedance in view of the size of the condensers I6 and 2 it may be obtained that the tuned circuit 3 forms the diagonal of a bridgeconnection, whose other diagonal is formed by total impedance 22 at whose ends the disturbing voltage is set up.

When'determining the size of the condenser 16 the natural capacity of the antenna need not be considered, since the natural capacity thereof is many times greater than the capacity of the condenser I6 which as a rule is not greater than 10 to 30 centimetres.

In the bridge circuit shown in Figure 3 a bridge connection is formed by the capacities 2, H, I6 and the impedance 22. One of the diagonals of the bridge is formed by the tuned circuit 3. The other diagonal is formed by the total impedance 22 at whose ends the disturbing voltage is set up. In accordance with the invention proper choice of the value of the impedance 22 with respect to the values of condensers l6 and 2 provides a balance with regard to the disturbing voltage at the ends of the impedance 22 with the result that the disturbances do not reach tuned input circuit 3 of tube 4. The points A and B are diagonal points of the bridge and points K and E are the other points of the bridge. It should be noted that points K and E of the bridge correspond to the points 18 and IQ of Figure 2.

I claim: 7

1. In a radio signalling system, an antenna circuit having natural capacity to ground, an amplifier tube comprising an anode, a cathode and at least one grid electrode, a capacity for connecting said antenna to said grid electrode, a connection including a capacity and an impedance for connecting said grid electrode to ground, said three capacities and said impedance forming a balanced bridge circuit, said grid electrode being also connected to ground through the natural capacity of said antenna, a signal input circuit connected between the grid electrode and the cathode and means for connecting the cathode to a point on said impedance such that the signal input circuit is effectively in a diagonal of the balanced bridge circuit.

2. In an amplifier circuit comprising a space discharge device having anode, cathode and grid electrodes, a source of oscillations to be amplifled comprising means having a capacitive connection to ground, means for connecting a grid electrode of said device to said source through a capacity, means for grounding the cathode of said device through an impedance and a tuned circuit connected between said grid electrode and said cathode whereby there is formed a circuit constituted by said capacitive connection, said connecting capacity, said tuned circuit, said impedance and said ground through which disturbances may be impressed upon the grid of said device and means for rendering said disturbances inefiective comprising a bridge circuit an arm of which comprises an auxiliary condenser, an adjacent arm being formed by a balancing impedance, the diagonal connecting said two arms including the tuned circuit and the grid cathode circuit of the device in parallel.

3. In a signalling system, at least one space discharge device, an energizing circuit therefor including a commercial network, said space dis charge device being susceptible to disturbances originating in the network through any inherent impedance of the ground connection, said space discharge device being provided with an anode, a cathode and a signal grid, a source of signalling energy, means for connecting said source to the signal grid, said signalling source being effectively connected to ground through a capacitive path, a connection including a capacity and an impedance for connecting the signal grid to ground, said signal grid being also connected to ground through said capacitive path and means for connecting the cathode to a point on said impedance, said connections forming a balanced bridge circuit with the grid cathode circuit of the space discharge device forming a diagonal of the bridge and the energizing circuit the other diagonal whereby disturbances originating in the commercial network supply system are substantially prevented from being impressed upon the gridcathode circuit of the space discharge device.

4. In a radio signaling system, an electronic tube provided with an anode, a cathode and a grid electrode, an input circuit for said tube comprising a connection between the grid electrode and the cathode, said connection including a tunable circuit, an antenna system having natural capacity to ground and means including a capacity device for connecting said antenna system to the grid electrode, a connection between the cathode of said tube and ground including an impedance element, a second connection be- :1

tween the grid electrode and the cathode including a capacity device in series with an impedance element connected in the order named, said two first named capacities, said two impedance elements and said last-named capacity forming a;

a balanced bridge circuit one arm of which comprises the first two capacities in series, an adjacent arm being formed by the last named capacity, the other adjacent arm being formed by the first impedance device and the opposite arm by the other impedance device, the input circuit of the tube being in a diagonal of the bridge.

5. In a radio signaling system, an electronic tube provided with an anode, a grounded cathode and a grid electrode, said tube being of the type which is energized through current derived from a commercial current network, an arrangement for preventing disturbances originating in the network from being impressed upon an input electrode of the tube through any inherent impedance in the ground connection, comprising an input circuit for said tube comprising a connection between the grid electrode and the cathode and including a tuned circuit, an antenna system having natural capacity to ground and means including a capacity device for connecting the antenna system to the grid electrode, a connection between the cathode of the tube and ground including an impedance element, a second connec- JOHAN WILLEM ALEXANDER. 

